Innervation is extremely important for maintaining the functionality of almost every part of the human body. Innervation of the gastrointestinal (GI) tract is extremely important for smooth muscle cells to maintain their phenotype and to perform their motor function, i.e., generating the forces necessary for fluid movement through the GI tract.
Upon nerve injury, muscle atrophy is commonly observed. Patients who suffer from different degrees of paralysis of the gut whether due to aging or diabetes (gastroparesis) also lack the neural elements as well as exhibit muscle atrophy. Similarly, children born with aganglionic gut disorders (e.g., Hirschsprung's disease) exhibit nerve degeneration.
Tissue engineering has been proposed to restore the function of diseased or damaged GI tract components by replaced degenerative muscle with new muscle structures. However, regenerating muscle using extracellular matrices seeded with muscle or muscle progenitor cells can replace a maximum about 10 percent of the lost muscle mass. Even with strenuous rehabilitation there is typically only about 30 percent recuperation of force generation.
For example, a functional gastric mucosa has been reported using gastric epithelial organoid units seeded on composite PLGA meshes to replace the native stomach of rats. The regeneration of stratified smooth muscle layers with the proper orientation, however, remains a challenge. Moreover, restoration of functional motility was not demonstrated in prior studies, highlighting the biggest challenge yet in functional tissue engineering of the GI neuromusculature.
Reconstruction of the stomach by tissue engineering is also a challenge. Aspects of bladder tissue engineering, whereby de-novo bladder reservoirs are manufactured with a variety of biomaterials, have been proposed as templates to re-engineer the musculature of the stomach. In stomach reconstruction, implantable gastric stimulation units, already commonly used in bariatric surgeries and in gastroparesis to stimulate enteric neurons or simulate gastric electrical rhythm, have been proposed as building blocks for stomach reconstruction. A report by Micci et al. demonstrates that the transplantation of CNS-derived neuronal progenitor cells can repopulate nitrergic neurons as well as improve gastric function in the pylorus of a rodent model of gastroparesis.
Tissue engineering also offered a possible advance to the bowel lengthening surgeries commonly carried out in short bowel syndrome. Collagen sponge scaffolds seeded with autologous smooth muscle cells have been successfully implanted as patch grafts in canine models. These patch grafts regenerated the mucosal and intestinal epithelial layers along with the villi structures. The major problem encountered with these grafts, however, was shrinkage. Dunn et al. used pseudo-tubular structures formed from collagen sponge scaffolds seeded with intestinal smooth muscle cells. The tubular structures were neovascularized within a month after prevascularization in the omentum. Unfortunately, these techniques did not regenerate the enteric neuronal layers, and the smooth muscle cells demonstrated a phenotypic switch to their non-contractile forms.
Tissue engineered small intestinal constructs, likewise, have not achieved the alignment of the smooth muscle cells or their innervation that appears to be crucial to generating appropriate force and motility to facilitate nutrient absorption.
Regeneration of colon segments is similarly elusive. The colon is contiguous with the small intestine, facilitating water absorption and excretion of stool. Loss of colonic segments by surgical resections e.g., to treat aganglionosis (Hirschsprung's Disease) or inflammation significantly alters colonic motility. Disruption of colonic motility alters transit time, resulting in constipation or diarrhea. The idiopathic nature of some of these disease states poses a strong demand for in vitro tissue engineered models of colon, where investigations can be carried out on individual components (smooth muscle, enteric neurons, interstitial cells and mucosa) to understand alterations in pathophysiological conditions. Moreover, alterations in peristalsis and segmental contractions of the colon have direct implications on an individual's quality of life.
Recently, Vacanti et al. reported a tissue engineered colon construct using composite poly lactic and glycolic acid polymers seeded with organoid units isolated from the sigmoid colon. They demonstrated that the tissue engineered conduits have significant absorptive capacity when implanted into animals, but there was no direct measurement of peristalsis or motility.
Phasic neuromuscular structures of the GI tract contain orthogonal layers of smooth muscle, interlaced with enteric neuronal plexuses. They are also associated with the interstitial cells of Cajal (ICC) and the specialized mucosal layers. Propagating peristaltic waves define the phasic nature of this neuromusculature. Peristaltic waves encompass contraction and relaxation of both the circular and longitudinal smooth muscle layers. The neuronal components as well as the ICC generate electrical activity for the coordination of peristalsis. This activity is coupled with intracellular biochemical events in the smooth muscle layers to regulate gut motility. These mechanisms are additionally segmentally modulated by the release of different neurotransmitters from the enteric neuronal plexuses as well as the electrical activity from the ICCs.
In a recent study by Pan et al., neural crest progenitor cells isolated from neonatal rats were transplanted into the distal colon of a rat model of Hirschsprung's Disease. These cells differentiated into neurons and glia in the host colon. They also demonstrated rescue of neuronal mediated motility in the aganglionic host colon. Metzger et al. demonstrated that adult human gut derived enteric progenitor cells can repopulate segments of human aganglionic colon grown in organotypic cultures.
Although significant advances have been made in tissue engineering of phasic neuromuscular structures, many gaps exist in proposed techniques for regeneration of functional smooth muscle and enteric neuronal plexuses. Accordingly, there exists a need for better techniques for innervated engineered tissues.